Paraxylene is a valuable petrochemical precursor for the production of terephthalate polymers such as polyethylene terephthalate. Polyethylene terephthalate is the polymer in several large scale end uses including polyester fibers and PET bottle resin. The large market for paraxylene for fiber and PET resin creates a need for large scale production and purification facilities for paraxylene. Paraxylene (PX) is typically isolated in refineries from the reformate produced by catalytic reformers although it can come other sources such as pygas. Catalytic reformers produce xylene molecules by the dehydrocyclization of straight and branched chain paraffins, the dehydrogenation of naphthenes, and to a small extent the dealkylation of polyalkylaromatics. The C.sub.8 aromatic portion of the reformate contains more than just paraxylene. The other aromatic isomers present in C.sub.8 boiling range reformate are orthoxylene (OX), metaxylene (MX), and ethylbenzene (EB). These isomers are difficult or impossible to separate from the paraxylene by distillation because they have close boiling points.
Two methods that have been developed to isolate the paraxylene (PX) from the other C.sub.8 isomers are crystallization and adsorption. Continuous simulated moving bed adsorption is currently used in most grass roots PX separation plants. Examples of such PX adsorption processes include Parex (UOP) and Eluxyl (IFP). These processes utilize an X or Y zeolite adsorbent in combination with either toluene or para-diethylbenzene as a desorbent. Selection of a desorbent is an important consideration and has significant impact on the design and economics of the process. In general the desorbent should be adsorbed about as well as the feed components on the selected adsorbent. If the desorbent is adsorbed too tightly to the adsorbent, an excessive fraction of the total capacity of the adsorbent is occupied by the desorbent. This effectively reduces the portion of the adsorbent that is available to adsorb and separate the C.sub.8 aromatic isomers, and increases the amount of adsorbent that must be used. If the desorbent is not adsorbed tightly enough large amounts of desorbent must be used causing a high desorbent to feed ratio. A high ratio of desorbent to feed results in the need for larger extract and raffinate distillation columns. The extract column separates the PX product from the desorbent. In a PX process the raffinate column separates the desorbent from the other C.sub.8 aromatic isomers. These distillation columns are a major part of the expense of a PX separation process. They are both a large portion of the capital cost of the process and cause a large portion of the operating cost of the PX separation plant. Thus it is generally best to minimize the amount of desorbent used in the process. Another important parameter of the desorbent is its boiling point. Since the desorbent is separated from the raffinate and the extract by distillation it is desirable to select a desorbent with a boiling point that is substantially different from the extract and raffinate boiling ranges. The closer the desorbent boiling point to the PX extract or C.sub.8 aromatic raffinate boiling range, the larger the column required and the higher the utility costs to operate the column. Also it is important to select a desorbent that does not form an azeotrope with any of the feed components in order to have a clean separation.
As discussed above adsorbents used for commercial PX separation from mixed xylenes typically comprise X or Y zeolites. X or Y zeolites are typically used because they have a relatively large adsorption capacity, are commercially available on a large scale, and can be formulated to adsorb PX relatively stronger than all three other C.sub.8 aromatic isomers, EB, OX, and MX as exhibited by the selectivity. To a great extent the properties desirable for a good adsorbent are a compromise and require the careful balancing of the properties of the adsorbent or the adsorbent/desorbent pair. For example the PX selectivity of X or Y zeolites relative to the other C.sub.8 aromatic isomers is only satisfactory.
Prior Patents that discuss the preparation and use of some of the preferred adsorbents useful in the present invention include U.S. Pat. No. 4,826,667 (SSZ-25), U.S. Pat. No. 4,439,409, and U.S. Pat. No. 4,954,325. The '667 patent to Zones et al. discusses preparation of SSZ-25 using an adamantane based template. The '667 patent also discloses on column 13 line 6 that "SSZ-25 can also be used as an adsorbent. . . ." However, the '667 does not provide any elaboration or disclosure as to the use of SSZ-25 as an adsorbent.
An adsorbent having improved PX selectivity relative to the other C.sub.8 aromatic isomers can have substantial economic advantages over prior art processes The process of the present invention provides just such advantages.